We propose to systematically study the molecular mechanism of polyethylene glycol (PEG) induced liposome and cell fusion. We found PEG destabilized bilayers and induced extensive aggregation and fusion among liposomes, and that fusion between erythrocyte membrane ghosts occurred only if the membranes had been resealed. We want to further define factors contrlling PEG-induced fusion of liposomes, membrane vesicles and cells in order to deduce the nature of PEG-membrane interaction. Large, single wall vesicles of phosphatidylcholine and phosphatidylserine, reconstituted membrane vesicles containing these lipids and integral proteins from erythrocyte membranes, erythrocyte membrane vesicles and ghosts, as well as intact erythrocytes will be used in our experiment. Aggregation and fusion will be measured by light scattering, cell sorting and microscopy. Morphology and molecular organization at intermediate stages of fusion will be monitored by rapidly-quenched freeze fracture electron microscopy, x-ray diffraction and NMR. The influence on aggregation and fusion by cations, mechanical stress and chemical disruption of bilayers will be analyzed to determine the effects of bilayer charge neutralization and structural defects. The fusion efficiency of both commercial and purified PEG, as well as those of other polymers will be compared. The importance of osmotic swelling and cytoskeletal control of cell fusion will be assessed. The knowledge gained from this study will form a scientific basis, and help to improve this widely used cell fusion procedure.